gve: Add RX context.

This refactor moves the skb_head and skb_tail fields into a new
gve_rx_ctx struct. This new struct will contain information about the
current packet being processed. This is in preparation for
multi-descriptor RX packets.

Signed-off-by: David Awogbemila <awogbemila@google.com>
Signed-off-by: Jeroen de Borst <jeroendb@google.com>
Reviewed-by: Catherine Sullivan <csully@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>